The present invention relates to the electrical arts and more specifically to active cascode amplifier circuits.
FIG. 1 is a prior art active cascode amplifier as shown and described in U.S. Pat. No. 5,039,054 which is incorporated herein by reference in its entirety. Active cascode amplifiers have both an output stage and an input stage. In FIG. 1 the input stage is represented by a Metal-Oxide-Semiconductor-Field-Effect-Transistor (xe2x80x9cMOSFETxe2x80x9d) transistor Q1 and the output stage is represented by another MOSFET transistor Q2. Attached to the gate of the output stage, MOSFET Q2, is the output of an auxiliary amplifier. The auxiliary amplifier, which in FIG. 1 is an operational amplifier has a bias voltage present at the positive input and the negative input is electrically coupled to node X providing a feedback loop. The operational amplifier increases the output resistance of the active cascode amplifier. Assuming a constant current source, the increased output resistance increases the voltage gain.
The active cascode amplifier operates in the following manner. When the input voltage signal, Vin drops below the threshold voltage of the MOSFET, Q1 shuts off. In response, the voltage at node X begins to increase, which in turn decreases the differential voltage between the positive and negative terminals of the operational amplifier. As a result, the output voltage at the control terminal (gate) of the output stage transistor Q2 decreases eventually shutting off the output stage and causing the active cascode amplifier to slew. Given enough time, the operational amplifier will saturate toward a voltage which is close to ground. When the output voltage of the operational amplifier drops to such a voltage that the gate to source voltage is less than that of the MOSFET""s threshold voltage for turning on, the output stage transistor Q2 shuts off. The output voltage of the operational amplifier continues to fall until the voltage approaches ground. Thus, the gate to source voltage falls well below the threshold voltage. When Vin then increases and goes above the threshold voltage for the transistor, such that Q1 turns on, the auxiliary amplifier requires a period of time, referred to as a xe2x80x9crecovery time periodxe2x80x9d for the voltage at the gate of Q2 to increase such that Q2 turns on. The recovery time period is shown in the graph of FIG. 2. This recovery time poses problems for devices which require quick circuit operation. The length of the recovery time is proportional to the difference between the gate voltage when Q2 is operational and the voltage at the gate when Q2 is off.
One solution to this problem known in the prior art is the inclusion of a trickle current source positioned at node X as shown in FIG. 3. The trickle current source provides a trickle current and thus a current flow path even when the input stage MOSFET Q1 is off. This trickle current causes the output stage transistor Q2 to remain in a partially on state. Since Q2 is in a partially on state, the recovery time is decreased as shown in the graph of FIG. 4 as compared to the graph of FIG. 2. One drawback of this configuration is that the trickle current source is constantly active drawing power and the power drain provides no increase to the speed of the amplifier.
In a first embodiment of the invention there is provided an active cascode amplifier circuit which includes an active cascode amplifier and an amplitude limiter. The active cascode amplifier includes an input stage, an output stage and an auxiliary amplifier and receives in a voltage input signal and outputs a voltage output signal wherein the active cascode amplifier amplifies the input voltage signal. The auxiliary amplifier is provided within the circuit to increase the gain of the cascode amplifier and has an associated output.
When the input stage shuts off, due to a decrease in the input voltage signal, the auxiliary amplifier""s output voltage falls and the amplitude limiter becomes active and holds the voltage at the output of the auxiliary amplifier to a preset voltage in order to decrease the recovery time for turning the output stage on when the input voltage increases and turns the input stage on.
The voltage at the output of the auxiliary amplifier provides a voltage to a control terminal of the output stage. When the output voltage of the auxiliary amplifier falls below a threshold voltage for the output stage, the output stage shuts off. The voltage at the control terminal would continue to fall, but for the amplitude limiter circuit. Thus, by preventing the voltage at the output of the auxiliary amplifier from falling below a preset limit, the recovery time to pull up the voltage at the control terminal and to turn the output stage on is decreased when the input stage transitions from an off state to an on state.
The amplitude limiter is a circuit which may be formed with circuitry including but not limited to a diode, a MOSFET, a JFET, or a bipolar transistor.
In one embodiment, the active cascode amplifier operates in a singe ended mode. In another embodiment, the active cascode amplifier operates in a differential mode. In further embodiments, the active cascode amplifier may be a folded active cascode amplifier which operates in a single ended or a differential mode.
The active cascode amplifier may be formed from any of a number of electrical components including but not limited to MOSFETs, JFETs, bipolar transistors, diodes and any combination thereof.
When the active cascode amplifier circuit is implemented in which the input and the output are provided as differential signals, a plurality of amplitude limiters are provided in the active cascode amplifier circuit. Each of the amplitude limiters operate in much the same way as for the single ended mode, in which the amplitude limiter holds the voltage level at the output of the auxiliary amplifier and does not allow the voltage to fall.
In a further embodiment the amplitude limiter becomes active when the output stage begins to shut off.